An examination of the reactivity of zinc di-alkyl-di-thiophosphate in relation to its use as an anti-wear and anti-corrosion additive in lubricating oils

The reactivity of the lubricating oil additive zinc di-alkyl-di-thiophosphate (ZDDP) was investigated by molecular orbital techniques. Semi-empirical quantum chemistry methods were used to model the structures of some of the complexes expected to form during the reaction of the additive with an oxygen rich steel surface. The results showed that the most stable of the complexes resulting from these reaction are those in which a short S-O bond is formed, thus suggesting that the sulphur atom is the most likely point of attack during the surface reaction.